Microporous polyolefin membranes are widely used for various applications such as battery separators used for, e.g., lithium ion batteries, nickel-hydrogen batteries, nickel-cadmium batteries, polymer batteries, electrolytic capacitor separators, various filters such as reverse osmosis filtration membranes, ultrafiltration membranes, microfiltration membranes, moisture-permeable, waterproof clothes, medical materials, etc. When the microporous polyolefin membrane is used for battery separators, particularly lithium ion battery separators, its performance largely affects the properties, productivity and safety of the batteries. Accordingly, the microporous polyolefin membrane preferably has excellent mechanical properties, heat resistance, permeability, dimensional stability, shutdown properties, meltdown properties, etc.
In general, microporous membranes composed only of polyethylene have low meltdown temperatures, while microporous membranes composed only of polypropylene have high shutdown temperatures. Accordingly, it may be advantageous to produce microporous membranes comprising both polyethylene and polypropylene as main components for battery separators. As used herein, shutdown temperature means the lowest temperature at which the membrane loses its permeability with the micropores collapsing by deformation of the material and therefore stopping the battery reaction. As used herein, meltdown temperature means the lowest temperature above shutdown temperature up to which the separator maintains its integrity and resists rupture and/or breakage.
For instance, JP2002-321323A proposes, as a microporous polyolefin membrane having allegedly excellent safety and strength, a microporous polyolefin membrane having a microporous membrane A comprising polyethylene and polypropylene as indispensable components, and a microporous polyethylene membrane B, which are integrally laminated to have a three-layer structure of membrane A/membrane B/membrane A or membrane B/membrane A/membrane B.
WO 2004/089627 proposes laminated polyolefin membranes constituted by two or more layers comprising polyethylene and polypropylene as indispensable components, a surface layer on at least one side containing more than 50% and 95% or less by mass of polypropylene, and the polyethylene content in the entire membrane being 50% to 95% by mass.
However, the microporous polyolefin membranes of these references fail to have good shutdown properties and pin puncture strength.
Accordingly, it would be desirable to have microporous membranes for battery separators comprising polyethylene and polypropylene and having well-balanced shutdown properties, meltdown temperatures, permeability and pin puncture strength as well as batteries made with such separators having enhanced and well balanced capacity, cyclability, discharge properties, heat resistance, compression resistance, productivity.